Wafers used in semiconductor manufacturing are typically inspected for defects such as, for example, surface imperfections, particles, or irregularities in thickness. These defects may impair operation of a semiconductor device. As each wafer processing step is expensive, manufacturers usually inspect the wafer between processing steps to determine whether the wafer should be reworked or scrapped.
Edges of a wafer can pose challenges during semiconductor manufacturing or inspection. Semiconductor manufacturers try to include as many semiconductor devices on a wafer as possible. Thus, inspection of the wafer edge may be needed to determine whether the edge can form usable devices. Devices near the wafer edge also may be inspected to determine yield or process control problems. However, wafer edges may be rounded or have irregular shapes. These rounded or irregular shaped edges can make inspection difficult.
Use of a semicircular scan of the wafer edge may result in images that are out of focus or unclear because the wafer edge may have an irregular, rather than perfectly semicircular, shape. This results in the optical head used for imaging being off-normal or defocused with respect to the edge profile over portions of the wafer surface or edge. For example, phase imaging over an irregular surface with a semicircular scan can result in a signal that is degraded by light and dark bands caused by variations in the angle of incidence of the imaging beam or defocus. Thus, sensitivity to defects is degraded.
Therefore, what is needed is an improved wafer edge inspection system and technique.